Thermodynamic, chemical, and electrochemical studies of Aloe ferox Mill extract as a naturally developing copper corrosion inhibitor in HCl solution

Copper can be susceptible to corrosion in acidic cleaning solutions for desalination system, especially if the solution is highly concentrated or if the cleaning process involves extended exposure to the acid. In the current work, Aloe ferox Mill (AFM extract) can be used as a natural origin corrosion inhibitor for copper in 1.0 M HCl solution. The corrosion mitigation qualities of AFM extract were assessed by means of electrochemical, gravimetric, and surface examinations. AFM extract is a mixed-type inhibitor, based on polarization research findings. The inhibitory effectiveness of AFM extract rises with concentration, reaching its maximum level (93.3%) at 250 mg L–1. The inclusion of AFM extract raises the activation energy for the corrosion reaction from 7.15 kJ mol–1 (blank solution) to 28.6 kJ mol–1 (at 250 mg L–1 AFM extract).


Experimental part Materials
In this experiment, copper samples with composition (%wt) copper 98%, Phosphorus 0.04%, Carbon 0.005, Silicon 0.2, Fe 0.18% and Zn 1.57% were evaluated.Before evaluation, the copper specimen was washed with a solution of ethyl alcohol and filtered water, and it was sanded with emery sheets varying within grade between 600 and 1200.1.0 M HCl solutions have been generated for each research employing water that had been deionized and analar class HCl (Merck & Co).AFM extract (Aloe ferox Mill) was obtained from Egyptian company.
Soxhlet extraction method was used involves continuous extraction using an ethanol (purity 99%) solvent, at elevated temperatures.The AFM extract was totally soluble in both water and hydrochloric acid.

Electrochemical and chemical experiments
For the electrochemical tests, a potentiostat/3000-Gamry was being used.The working component of the tests was a copper plate with a functional area of 0.469 cm 2 , the counter component was a Pt strip, and the reference component was a saturated calomel electrode (SCE).The experiments were carried out in a double-walled cell.Tafel polarisation graphs were created by applying varying voltage ranging (± 0.250 V/SCE vs. OCP) to the copper anode using a scan rate of 1.0 mV s -1 .
Experiments using electrochemical impedance spectroscopy (EIS) were conducted at frequencies between 0.01 Hz and 100 kHz at an open circuit potential using voltage amplitude of 10 mV.
The electrochemical tests were carried out repeatedly to ensure their accuracy.
Copper sheets were divided into pieces sized 1.8 cm × 1.0 cm × 0.05 cm in order to assess gravimetric tests.The solutions being examined (100 ml) were immersed in the copper plates.The standard procedure G1-03-2017-e1 ASTM was utilized to determine the gravimetric 12 .Each of the samples was immersed in liquids for a whole day.Three replicates of the experiments were conducted, and the average loss of mass was ascertained.
The following equation was used for calculating the copper (C R ) corrosion rate 13 : (C R = corrosion rate, W = mass loss (mg), A = cu surface area (cm 2 ), t = time (h)).
Objectives were made to do many tests using a water bath with temperature controls at different temperatures (298, 313, 323, and 333 K).

Surface investigations
The copper surface was analyzed using a range of surface characterization techniques.Throughout the tests, the electrodes were immersed in 1.0 M HCl solution incorporating AFM extract (250 mg L -1 ) for a period of three hours.They were washed with water that was distilled and dried out after being removed from their cells.The surface of copper was examined using scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) (system ZEISS EVO).FTIR analysis was performed on both the pure AFM extract and the corrosion products using Agilent FTIR device.

Electrochemical studies
For 30 min, the open circuit potential (OCP) measurements for copper in 1.0 M HCl were identified both with and without the incorporation of AFM extract.The resulting curves are displayed in Fig. 1.Compared to the trends when AFM extract is present, the OCP in the blank solution is less dramatically altered to more negative values.This actually to cause by of corrosion products adhering to the copper surface.The examination of the OCP values in uncontrolled and inhibited liquids at the end of the tests allowed for the classification of the AFM extract as a mixed-type inhibitor, which has a stronger impact on the cathodic operation 14 .
Figure 2 shows the Tafel polarization plot regarding copperin 1.0 M HCl with various dosages of AFM extract at 298 K.The polarization analysis demonstrated that each of the cathodic and anodic currents is changed when the amount of AFM extract is increased.Table 1 gives the Tafel polarization results.There is not a noticeable trend in the corrosion potential (E corr ) measurements.The AFM extract's mixed type behavior was supported by   www.nature.com/scientificreports/ the difference in E corr values between the acid solutions including the extract and the blank acid solution, which showed to indicate below 85 mV 14 .
It is evident that the anodic and cathodic processes' mechanisms were unaffected by the addition of AFM extract to the corrosion surroundings, since no significant modifications were seen in the cathodic Tafel slop (β c ) or anodic Tafel slop (β a ) [15][16][17] .
The corrosion current density (j corr ) results show a considerable drop with the addition of AFM extract.This suggests that in 1.0 M HCl solutions, AFM extract inhibits copper anode corrosion 18 .Using the following formula, the AFM extract's inhibitory ability (η j %) was calculated 19 : (j corr(0) = corrosion current density in blank solution, j corr = corrosion current density in the presence of AFM extract).
The inhibitory capacity of AFM extract increases with concentration, reaching its highest value (93.3%) at 250 mg L -1 .These results support the significant inhibitory action of AFM extract regarding copper corrosion in 1.0 M HCl.
EIS tests were conducted to more thoroughly examine the impact of AFM extract on the corrosion behavior of copper in 1.0 M HCl. Figure 3 displays the acquired results (Nyquist diagram).Table 2 summarizes the EIS parameters that were determined by fitting, as indicated by Nyquist diagram.It is evident from examining the Nyquist diagram (Fig. 3) that the semicircle width raises with increasing AFM extract concentration.Corrosion rate is lowered as a result.
The inset of Fig. 3 illustrates the proposed equivalent circuit that was used to analyze and simulate the obtained graphs.The charge transfer resistance is represented by R ct , the solution resistance by R s , and the ideal capacitor is replaced by the constant phase element (Q dl ) 20 .The accuracy of the fitting procedures is shown by the relatively low goodness of fit (χ2) results in Table 2.The data displayed in Table 2 demonstrate that when (2) η j % = j corr(0) − j corr j corr(0) × 100,  Furthermore, as the concentration of AFM extract increased, values of Q dl decreased.This is related with the adsorption of AFM extract molecules on the copper surface leading to decrease exposed copper surface to aggressive ions.The following equation was utilized to calculate the inhibitory efficacy (η R %).
(R cto is recoded in 1.0 M HCl solution and R ct is recoded in the presence of AFM extract).Table 2 lists the calculated inhibition efficiencies.The results derived by polarization (Table 1) agree with the computed EIS values (Table 2).
The adsorption of AFM extract on the copper surface can indeed play a crucial role in inhibiting corrosion in HCl solution [21][22][23] .The adsorption can occur through various mechanisms, such as chemisorption or physical adsorption.Chemisorption involves a strong chemical interaction between the inhibitor and the metal surface, while physical adsorption involves weaker, non-specific interactions.When the AFM extract adsorbs onto the copper surface, it forms a protective barrier that acts as a physical or chemical shield against corrosive species.This barrier can inhibit the diffusion of corrosive ions or molecules, reducing their access to the copper surface.The effectiveness of corrosion inhibition is often related to the surface coverage of the AFM extract molecules on the copper surface.Higher surface coverage leads to greater protection by forming a denser and more uniform layer, preventing or reducing the contact between the corrosive environment and the copper surface.

Chemical studies
As can be observed from the results shown in Table 2, the method of gravimetric measurements was used to assess the weight loss rate and extent of protection for copper in 1.0 M HCl solution in both the presence and the absence of AFM extract at 298 K.The equation below was utilized to quantify the η W % inhibitory effectiveness of AFM extract 24 : (C R0 = corrosion rate in blank solution, C R= the corrosion rate in the presence of AFM extract).The C R and η W % at varying quantities of AFM extract are outlined in Table 3.As the concentration of AFM extract rises, the value of C R keeps reducing.This decrease in C R as AFM extract concentration rises is indicative of a tendency towards increased surface coverage of the copper via AFM extract molecules.The maximum inhibitory efficiency (η W % = 91.0)was seen at a 250 mg L -1 concentration of AFM extract.
Many studies investigated the inhibitory effects of various extracts on copper corrosion inside HCl solution.We conducted a comparison of our findings with earlier research published on copper corrosion inhibition in acidic conditions.Wu et al. 25 investigated the corrosion inhibitory effects of psidium guajava linn leaves extract on copper in a sulfuric acid solution.Once the amount of psidium guajava linn leaves extract achieves 600 mg/L, it may suppress corrosion by 95%.Kathiravan et al. 26 used electrochemical methods to study the protection of Ruellia Tuberosa L leaves extract against copper in 0.5 M HCl.The extract at a concentration of 0.18 g/L provided 87.01%protection.Ahmed and Zhang 27 have evaluated the inhibitory action of Atriplex leucoclada extract by experimental investigations for copper in1 M HCl.At a concentration of 8 g/L, Atriplex leucoclada extract had the highest inhibitory performance (91.5%).Jmiai et al. 28 studied the the protection of Jujube shell extract in 1 M HCl utilizing chemical and electrochemical strategies.The findings showed a maximal inhibitory effectiveness of 91.0% at 1 g/L.Compared to the majority of the values given in the published research, AFM extract has a distinct superiority in considerations of both dose and matching efficiency, showing that AFM extract is an efficient corrosion inhibitor, and hence its use in copper protection is critical.

Thermodynamic studies
Thermodynamic studies provide valuable insights into the fundamental mechanisms of corrosion inhibition of AFM extract [29][30][31] .
Table 3. Gravimetric parameters of copper at 298 K in 1.0 M HCl solution with and without AFM extract.www.nature.com/scientificreports/In 1.0 M HCl solution, both with and without AFM extract (250 mg L -1 ), Table 4 shows the C R and η W % for copper as dependent variables of temperature (298 to 333 K).Evidence from the data indicates that when the temperature goes up, the C R of copper in acid solution, whether blank or controlled tends increases.A change in the adsorption/desorption equilibrium towards the de-sorption of AFM extract from the copper surface as well as the roughness of the copper surface can explain this tendency 32 .As the temperature goes up, the η W % continuously decreases (Table 4), suggesting a physisorption mechanism 33 .The fact that increasing temperatures have minimal impact on η W % indicates that the AFM extract can be seen of as an efficient inhibitor, especially at high temperatures.Thermodynamic parameters, such as enthalpy (∆H*) and entropy (∆S*), can be determined to assess the nature and mechanism of the corrosion inhibition process.Thermodynamic parameters are extracted from Eqs. ( 5) and ( 6) 34 .
(h = 6.6261 10 −34 m 2 kg s −1 , R = gas constant, N = 6.2022 1023 mol −1 , and A = constant).The E a was determined using an Arrhenius diagram (Fig. 4) with or without of the 250 mg L -1 AFM extract.When AFM extract is added, the E a increases to 28.6 kJ mol -1 from 7.15 kJ mol -1 (blank solution).Higher activation energy in the presence of AFM extract hinders down copper corrosion.The dimension of the double layer increases as a result of the adsorption of AFM extract on the surface of the copper, raising the energy Table 4. Gravimetric parameters for Copper in 1.0 M HCl solution with and without AFM extract (250 mg L -1 ) at various temperatures.www.nature.com/scientificreports/barrier required to start the corrosion process.This was connected to the molecules of the AFM extract having favorable physical adsorption 35,36 .The values of ΔH* and ΔS* were obtained using the transition diagram (Fig. 5).When AFM extract is added, the ΔH* increases to 26.01 kJ mol -1 from 4.53 kJ mol -1 (blank solution).The positive magnitude of ΔH* indicates the endothermic process of copper oxidation in acidic solution 37,38 .From − 194 J mol −1 K −1 (blank solution) to − 174 J mol −1 K −1 , there was a minor variation in the ΔS*.In addition, it is possible to attribute the shift in ΔS* from a negative value in the blank solution to a lower negative value in the AFM extract solution as a result of the rise in H 2 O entropy caused by H 2 O de-sorption over the copper surface in the presence of AFM extract 39 .www.nature.com/scientificreports/AFM extract into the corrosive solution (Fig. 7) greatly decreased its corrosive properties and avoided corrosion of the copper surface, as evidenced through a significantly smooth copper surface being nearly totally coated by AFM extract molecules.The associated EDAX includes O K (3.2%), N K (2.6%), S K (0.8%), and Cu K (93.4%), indicating that the AFM extract molecules adsorb on the copper's surface.The FTIR spectra (see Fig. 8a) derived from the pure AFM extract revealed peaks related to O-H (3412 cm −1 ), C-O stretching (1630 cm −1 ), benzene ring (1484 cm −1 ), C-N stretching (1232 cm −1 ), and C-O-C stretching (1035 cm −1 ).The results thereof revealed that the AFM extract contains nitrogen and oxygen in its functional groups, which is consistent with the chemical structures of common corrosion inhibitors.When comparing the AFM extract's spectrum to that of the solid corrosion result (Fig. 8b), shifts with regard to the benzene ring, O-H, C-N stretching, and C-O-C frequencies are seen.These findings pointed out that the surface of copper had absorbed AFM extract.The shifts in the spectra demonstrate that the functional groups in the components of the AFM extract interacted with copper during the contact.

Conclusions
Aloe ferox Mill (AFM extract) can be utilized in the present research as a naturally generated corrosion inhibitor for copper in HCl solution.Tests using electrochemistry, SEM/EDAX/ FT-IR and gravimetric analysis were employed to assess the AFM extract's ability to suppress corrosion.AFM extract has an inhibitory ability that rises with concentration and reaches its maximum value (93.3%) at 250 mg L -1 .The corrosion inhibition efficiency steadily drops with increasing temperature, pointing to a physisorption process.
Increasing the temperature has little effect on corrosion inhibition efficiency, suggesting that the AFM extract is an effective inhibitor at high temperatures.Copper corrosion gradually slows down when AFM extract is present because of the higher activation energy.In an acidic solution, the endothermic process of copper oxidation is shown by the positive magnitude of ΔH*.

Figure 1 .
Figure 1.Open circuit potential curves for copper in 1.0 M HCl without and with different AFM extract concentrations added at 298 K.

Figure 2 .
Figure 2. Tafel polarisation plot of copper in 1.0 M HCl without and with different AFM extract concentrations added at 298 K.

Figure 3 .
Figure 3. Nyquist curves of copper in 1.0 M HCl without and with different AFM extract concentrations added at 298 K (inset: equivalent circuit used to fit the EIS data).

Figure 4 .
Figure 4. Arrhenius plot for copper in 1.0 M HCl solution in the presence/absence of AFM extract (250 mg L -1 ).

Figure 6 Figure 5 .
Figure 6 illustrates a SEM photo of Cu after 3 h immersion in 1.0 M HCl without AFM extract.The a view of the Cu surface in 1.0 M HCl (Fig. 6) indicates that the top layer of the metal has been severely corroded by copper dissolve when exposed to this aggressive solution, resulting in numerous cracks and pits evident on the copper's surface.Cu-oxide is formed as a result of the chemical reaction of the copper's outer layer and the HCl solution.The corresponding EDAX includes O K (19.6%) and Cu K (80.4%), indicating the production of copper oxides CuO and/or cuprites Cu 2 O as products of corrosion on the copper's surface.whereas introducing 250 mg L -1 of

Figure 6 .
Figure 6.SEM and EDAX QUANTIFICATION for copper in 1.0 M HCl solution.

Figure 7 .
Figure 7. SEM and EDAX QUANTIFICATION for copper in 1.0 M HCl solution in the presence of AFM extract (250 mg L -1 ).

Figure 8 .
Figure 8. FTIR spectra of (a) AFM extract and (b) AFM extract adsorbed on copper in 1.0 M HCl solution in the presence of AFM extract (250 mg L -1 ).

Table 1 .
Polarization parameters for copper in 1.0 M HCl solution in the absence and presence of AFM extract at 298 K.

Table 2 .
EIS parameters for copper in 1.0 M HCl solution in the absence and presence of AFM extract at 298 K.AFM extract (